The technology of the present disclosure relates to a fixing device for fixing a toner image to a paper, an image forming apparatus provided with the fixing device, and an oblique motion restraint member for a fixing belt installed in the fixing device.
A fixing device for fixing a toner image to a paper is installed in an electro-photographic image forming apparatus such as a copier, a printer or the like. As a fixing method employed in the fixing device, a “thermal roller method” in which a toner image is fixed to a paper in a fixing nip formed between a pair of rotatable rollers is extensively used from the viewpoint of thermal efficiency and safety. In the meantime, there is a demand for the shortening of a warm-up time and the energy saving. Thus, attention is recently paid to a “belt method” in which a fixing nip is formed using a rotatable fixing belt installed around one or more rollers.
In the belt method, due to the misalignment of rollers around which a fixing belt is installed, it is often the case that a force acting outward in a direction of a rotation axis of the fixing belt is applied to the fixing belt during rotation of the fixing belt, thereby causing the fixing belt to be obliquely moved. If the oblique motion becomes severe, a problem of the fixing belt interfering with other members is likely to occur.
Thus, there is known a configuration in which an oblique motion restraint member is disposed at an outer side of a fixing belt in a direction of a rotation axis and is brought into contact with an end surface of the fixing belt to thereby restrain oblique motion of the fixing belt. However, if the oblique motion restraint member is brought into contact with the end surface of the fixing belt, the end surface of the fixing belt is repeatedly pressed by the oblique motion restraint member during rotation of the fixing belt, consequently generating a crack on the end surface of the fixing belt. This may lead to breakage of the fixing belt. In particular, a fixing-nip-adjoining portion of the end surface of the fixing belt is deformed along with the formation of the fixing nip. Therefore, the aforementioned crack is easily generated.
Under these circumstances, there has been proposed a configuration for avoiding the problems noted above. This configuration will be described below with reference to FIG. 7.
A fixing device 51 includes a fixing roller 52, a fixing belt 53 installed around the fixing roller 52, a pressing roller 55 pressed against the fixing belt 53 to form a fixing nip 54 between the fixing belt 53 and the pressing roller 55, and an oblique motion restraint member 56 installed at an outer side of the fixing belt 53 in a direction of a rotation axis Y. The oblique motion restraint member 56 includes a contact portion 57 capable of making contact with an end surface 59 of the fixing belt 53 and a breakage-preventing portion 58 installed radially inward of the contact portion 57. If the portion of the fixing belt 53 existing at the side of the fixing nip 54 becomes smaller in diameter along with the formation of the fixing nip 54, a fixing-nip-adjoining portion 59a of the end surface 59 of the fixing belt 53 moves from the contact portion 57 toward the breakage-preventing portion 58. Thus, the fixing-nip-adjoining portion 59a of the end surface 59 of the fixing belt 53 is prevented from being excessively pressed by the oblique motion restraint member 56.
However, the frictional resistance between the end surface 59 of the fixing belt 53 and the contact portion 57 of the oblique motion restraint member 56 is large because the contact portion 57 of the oblique motion restraint member 56 is formed into a planar shape and because the end surface 59 of the fixing belt 53 makes thorough contact with the contact portion 57 of the oblique motion restraint member 56. For that reason, even if the portion of the fixing belt 53 existing at the side of the fixing nip 54 becomes smaller in diameter along with the formation of the fixing nip 54, the fixing-nip-adjoining portion 59a of the end surface 59 of the fixing belt is expanded radially outward without moving from the contact portion 57 toward the breakage-preventing portion 58. In this state, if the fixing-nip-adjoining portion 59a of the end surface 59 of the fixing belt 53 is repeatedly pressed by the oblique motion restraint member 56 during rotation of the fixing belt 53, a crack is prematurely generated on the end surface 59 of the fixing belt 53. This may lead to breakage of the fixing belt 53.